Tens of thousands of anti-government protesters marched in Malaysia's capital on Saturday demanding the resignation of Prime Minister Najib Razak over his alleged involvement in a multi-billion dollar misappropriation scandal.

Protesters clad in yellow shirts marched through the heart of Kuala Lumpur -- bringing traffic to a standstill in several tourist spots -- to gather in front of the iconic Petronas Twin Towers after an initial plan to assemble at Independence Square was thwarted by police.

Turnout was lower than a similar rally last year, with news portal Malaysiakini estimating around 40,000 participants. As many as 200,000 people showed up last year at one point. Turnout for this year's rally may have been hurt by fears of violence and arrests of leaders from the pro-democracy group, Bersih, that organised Saturday's rally. Several opposition leaders were also detained just a day before the demonstration, along with Bersih leaders.

Police had said the rally is illegal and that they would not hesitate to use tear gas or water cannon if things got out of hand. State news agency Bernama said about 7,000 policemen will be on duty near the protest areas.

Still, spirits were high among those that gathered, with drums and vuvuzelas heard along with speeches, songs and chants by participants calling for a clean Malaysia and people power.

Former Malaysian premier Mahathir Mohamad, Najib's fiercest critic, joined protesters in front of the Twin Towers. "We are no longer a democracy, we are known as a kleptocracy. This is the time we work together to bring down this cruel government," said 91-year-old Mahathir.

Muhyiddin Yassin -- the former deputy premier who now leads a new party chaired by Mahathir -- was also present, leading the crowds in chants of "Step down Najib!"

The demonstration is unlikely to shake the prime minister, though, who has denied wrongdoing and weathered the crisis, consolidating power by cracking down on dissenters and curbing media groups and activists. "We are not here to bring down the country. We love this country! We are not here to tear down the government, we're here to strengthen it," Bersih deputy chair Shahrul Aman Shaari told the crowds gathered at the National Mosque.

Another Bersih leader Hishamuddin Rais was arrested on Saturday at the protest area, with police also issuing warnings to other participants. "Our country is being governed by clowns and crooks. So I'm here to protest against our prime minister," said artist Fahmi Reza, holding a poster of a clown-faced Najib.

In a speech uploaded on his website on Friday, Najib said the protesters were "a tool of the opposition".

"Their movement is deceitful. It is clear that these street protests are in fact the opposition disguised as an independent NGO working to unseat a democratically elected government," said Najib, who is in Peru to attend the Asia-Pacific Economic Cooperation (APEC) summit.

Azalina Othman Said, a minister in the Prime Minister's Department, on Saturday said it was unlawful for any party to try to unseat a democratically-elected government via street protests. Bersih rallies were also held in Kota Kinabalu and Kuching in East Malaysia on Saturday.

Corruption scandal

Najib has faced criticism since the Wall Street Journal reported last year that around $700 million from state fund 1Malaysia Development Berhad (1MDB) was diverted into the personal bank account of the prime minister.

Najib ran into further trouble this year when lawsuits filed by the US Justice Department in July said over $3.5 billion was stolen from 1MDB, which was founded by Najib, and that some of those funds flowed into the accounts of "Malaysian Official 1", whom US and Malaysian officials have identified as Najib.

Najib has taken steps critics say aim to limit discussion of the scandal, such as sacking a deputy prime minister, replacing the attorney-general and suspending newspapers and blocking websites.

Bersih chair Maria Chin Abdullah -- who was arrested on the eve of the rally -- has been detained under Malaysia's Security Offences (Special Measures) Act, or SOSMA as it is known, her lawyers said on Saturday.

SOSMA was introduced in 2012 to protect the country from security and extremist threats.

"Sosma has now been used against Bersih when it should only be used against genuine security cases or terrorists," co-founder of Lawyers for Liberty Eric Paulsen said on Twitter. "Her Sosma detention is clearly made in bad faith & not intended by Parliament."

Najib retains significant support within UMNO and from the long-ruling Barisan Nasional coalition.

Tensions

Fears of clashes between Bersih and a pro-Najib group called Red Shirts mounted earlier this week after the latter threatened to target Bersih supporters, though no major clashes were reported on Saturday during simultaneous demonstrations.

The pro-Najib group also rallied on Saturday, and local media reported one Red Shirt was detained for attempting to attack Bersih participants.

A six-week campaign by Bersih ahead of this weekend was marred by several violent confrontations with the Red Shirts, and anonymous death threats were sent to Bersih chair Maria.

Jamal Yunos, an UMNO member and leader of the Red Shirts, was also arrested on Friday before the protests began. He had warned of a repeat of racial riots in 1969 that killed hundreds in clashes between Malays and ethnic Chinese. Ethnic Chinese and Indian minorities formed the bulk of the Bersih rally, similar to last year.